Dehydration of a gas, for example a natural gas or a refinery gas, is a conventional operation. It allows the dew point of the gas to be controlled, to prevent the formation of hydrates or ice during transport or use of the gas; it can reduce the risk of corrosion, etc. . . . .
To this end, the gas is currently brought into contact with a hydrophilic liquid desiccant; of these, glycols are very widely used. Triethyleneglycol (TEG) is used most frequently in almost 95% of cases, because of its high affinity for water, its chemical stability and its low cost. However, for certain applications, monoethyleneglycol (MEG), diethyleneglycol (DEG) or tetraethyleneglycol (T4EG) may be preferred.
In a conventional gas dehydration unit using a liquid desiccant, for example glycol, as shown in the accompanying FIG. 1, the wet gas enters via line 1 at the bottom of an absorption column A1, operating under pressure, where it contacts a counter-current of liquid desiccant introduced overhead via line 3. During contact, the water contained in the gas is absorbed by the desiccant. The dehydrated gas leaves absorption column A1 overhead at high pressure via line 2. The desiccant charged with water leaves the bottom of column A1 and is sent via line 4 to the head of a regeneration unit R1 where it is used as a cooling fluid. After heat exchange, the desiccant charged with water is sent to a flash separation drum S1 where the pressure is lower than in absorption column A1. In some cases, the desiccant charged with water is first sent to the flash separation drum before using it as a cooling fluid at the head of regeneration unit R1. A large portion of the gas absorbed at high pressure by the desiccant is separated from the liquid phase in drum S1. The gas can either be discharged into the atmosphere via line 5 or used as fuel gas during the desiccant regeneration step, in which case it is sent to the burner of reboiler R2 of regeneration apparatus R1.
The liquid desiccant containing water, but separated from the gas absorbed at high pressure, leaves the flash separation drum via line 7. After passage through at least one heat exchanger E1, it is sent via line 7 to thermal regeneration apparatus R1, in which a portion of the water absorbed by the desiccant is vaporised and eliminated overhead via line 8, while the regenerated desiccant which leaves via line 3 passes through exchanger E1 and is sent via a pump P1 through cooler E4 then to the head of absorption column A1.
It is known, however, that the water cannot be completely separated from the desiccant using a thermal route at atmospheric pressure since the desiccant degrades at a temperature below its normal boiling point. As an example, TEG boils at about 285.degree. C., but a limit of 204.degree. C. is generally applied during regeneration to limit degradation. At this temperature, the purity of the regenerated TEG is close to 98.7% by weight.
When greater purity is desired for the liquid desiccant (glycol) in order to produce more effective dehydration of the gas, a conventional method consists of following the thermal reconcentration step by a stripping step using a gas which is dry or contains a small amount of water, for example a portion of the gas stream which has been dehydrated by the desiccant, as described in particular in United States patent U.S. Pat. No. 3,105,748.
A further technique consists of following the reconcentration step by a stripping step using a liquid stripping agent at ambient temperature and pressure and forming a heteroazeotrope with water. This configuration, which is described in French patent FR-B-2 698 017 in particular, comprises:
1. a reboiling step for the liquid desiccant charged with water; PA1 2. a desiccant distillation step comprising at least one distillation stage; PA1 3. a stripping step for the liquid desiccant which has been partially regenerated during steps 1 and 2, using the vaporised stripping agent; PA1 4. a step for condensing the vapour leaving distillation step 2, to generate two liquid phases, one being mainly water, the other being mainly stripping agent; PA1 5. heating the liquid phase which is rich in stripping agent from step 4, said heating regenerating a vapour phase which is richer in water than said liquid phase and a liquid phase which is depleted in water; PA1 6. returning the liquid phase constituted essentially by stripping agent from step 5 to step 3. PA1 a gas which always contains BTEX; PA1 a solution containing water and TEG, which can be regenerated without risking BTEX emissions. PA1 (a) a step for absorbing water and BTEX by contacting said wet gas with the liquid desiccant which has been regenerated in step (c), producing a dry effluent gas and a stream of liquid desiccant charged with water and BTEX; PA1 (b) a step for separating said charged liquid desiccant into a vapour containing mainly methane, water vapour and a portion of the BTEX, and a liquid phase containing mainly the liquid desiccant charged with water and BTEX; PA1 (c) a step for regenerating said liquid desiccant, comprising a reboiling zone and a distillation zone, in which the charged liquid desiccant is sent to said distillation zone, from which a vapour containing water and BTEX and said regenerated liquid desiccant are extracted, which latter is sent as the desiccant to the inlet to said absorption zone of step (a); PA1 (d) a step for condensing the vapour from said distillation zone, followed by separation into three phases: a gaseous effluent containing BTEX, a liquid hydrocarbon phase containing BTEX and an aqueous liquid phase; and PA1 (e) treating at least said gaseous effluent containing BTEX in a washing zone by absorbing the BTEX with a fraction of the regenerated liquid desiccant which is taken from a point in the process and returning said desiccant, having absorbed the BTEX, to a point in the regeneration zone of step (b), the gaseous effluent leaving said washing zone having been freed of BTEX.
In dehydration processes, when the treated natural gas or refinery gas contains aromatic compounds (BTEX): at least one of benzene, toluene, ethylbenzene and xylene), during the absorption phase, the desiccant--generally TEG--which is also a solvent for aromatic compound, becomes charged with BTEX.
Because of the boiling points of BTEX at atmospheric pressure, i.e., in the range 80.degree. C. to 144.degree. C., little of these compounds are separated from the desiccant in the flash separation drum described above, which operates at low pressure and high temperature. The majority of the aromatic compounds are separated from the desiccant when it is heated in the regeneration column.
The vapours emitted by a TEG reboiling unit can have a very high total aromatic content (more than 30%). By way of indication, a particular composition (treatment of a natural gas field at Whitney Canyon, Wyoming, United States) is given below (% by weight):
______________________________________ Water 45.2% Nitrogen 7.7% Benzene 4.6% Toluene 15.6% Ethylbenzene 0.9% Xylene 12.7% Other hydrocarbons 13.3% ______________________________________
The composition of the discharge varies depending on the nature of the gas to be treated, the temperature and the flow rate of the TEG circulating in the facility. This discharge must be reduced in order to comply with new regulations regarding the emission of toxic substances into the atmosphere. As an example, in the United States, the "Clean Air Act Amendment" of 1990 drastically reduces the acceptable levels of BTEX discharged into the atmosphere on American territory. All units discharging more than 100 tonnes/year of BTEX or 25 tonnes/year of any combination of these 4 compounds are monitored and regulated.
In order to comply with the new regulations on the emission of toxic substances into the atmosphere, the manufacturers concerned have modified existing gas dehydration units using the following techniques:
Vapour incineration, which can be carried out in a flame incinerator supplied with fuel gas produced by the unit, which has the disadvantage of requiring very high investment.
Vapour condensation to produce water and BTEX and gravity separation in a three-phase separation drum is described in detail in U.S. Pat. No. 3,867,736 and shown schematically in FIG. 2. In this technique, the gaseous discharges leaving overhead of thermal regeneration apparatus R1 are sent via line 8 to a condenser C1, usually an air-cooled exchanger. The various fluids leaving condenser C1 are sent to a three-phase separation drum B1 where a liquid phase containing mainly water is evacuated via line 11, and a liquid phase containing mainly hydrocarbons is extracted as a side stream via line 10, separation occurring under gravity. The gaseous phase leaving three-phase drum B1 via line 9 is composed of water vapour and contains a residual amount of hydrocarbons which frequently exceeds the environmental limits, as will be seen in Example 2 below.
An industrial process is known which uses two condensers like C1 and two three-phase drums like B1. Such a process can treat the vapours emitted by flash separation drum S1 and by regeneration column R1.
U.S. Pat. No. 5,209,762 describes an improvement over the above process which can eliminate the aromatics dissolved in the liquid water extracted from the three-phase drum.
In another technique, a primary condenser is installed in the vapour circuit, followed by a screw-type compressor. The non condensable vapours are reintroduced into the treatment unit.
In a further technique, a gas is dried and treated using a solvent composed of a glycol, N-methyl caprolactam and water. The concentration of the glycol (preferably TEG) is in the range 80% to 97%. This method is described in U.S. Pat. No. 4,479,811.
Finally, gas permeation has been described for this application, in U.S. Pat. No. 5,399,188. A mixture of water and TEG circulates inside a bundle of hollow fibres in a chamber. The wet gas containing BTEX is sent to the chamber. Only water mixed with glycol passes through the membrane. The following is recovered at the chamber outlet: